It is illegal to discriminate against applicants for being old. But two years ago, in Hunter's open house, the professor didn't know this, and he said they favor minds young enough to mold. Also, Iowa got sued a few years ago by an old guy who was rejected. The lawsuit revealed that the admission rate for people 40+ was lower than for younger people. So some people believe that MFAs are discriminating against old people, but at least in Fiction the general consensus is the discrimination is with people 37+.
Poets tend to go to MFA school shortly after ugrad. It is not clear if this is because schools are discriminating or because that is just poets tend to apply younger than fiction people.